The kinetics of the reactions between triphenylphosphine 1 and dialkyl acetylenedicarboxylates 2, in the presence of a NH-acid such as 5-nitroindazole 3,were studied. Corresponding kinetic parameters to all reactions were evaluated, with the second order rate constant (k) values calculated. Effects of solvent, temperature, and reactants (dialkyl acetylenedicarboxylates) structure and concentration were evaluated on the reaction rates. Theoretical studies were performed to evaluate potential energy surfaces for all structures participating in the reaction mechanism. For all reactions, the first step was recognized as the rate-determining step, on the basis of experimental and theoretical data. Quantum mechanical calculations were utilized to clarify how the ylides exist in solution as a mixture of two geometrical isomers (Z- and E-) -the issue of majority isomer.